Brief History of Astronomical Photography
Niépce and Daguerre
After a photograph was taken on the plate, it was developed by exposing the plate to a current of mercury vapor heated to a temperature of 75° Celsius. The vapor would adhere to the part of the plate which had been exposed to the light. The plate was then fixed by immersing it into sodium thiosulfate, which was used to dissolve the unused silver iodide, and finally rinsed in hot water to remove any remaining chemicals. The importance that photography could have in the field of astronomy was immediately realized. It would allow an accurate and easy recording of brightness, positions, spectra, and physical aspects of celestial bodies. However, these early photographic plates were not sensitive enough to image faint objects. The first daguerrotype of the moon was made by American physiologist and chemist John William Draper (at left; 1811-1882) in 1840, involving a full 20 minute exposure. The first star was not recorded until the night of July 16-17, 1850, when William Cranch Bond, the director of Harvard College Observatory, and J. A. Whipple, a photographer associated with Massachusetts General Hospital, took a daguerrotype of Vega. At right is an 1852 daguerrotype of the Moon taken by Whipple.
Wet Collodion Process
Mizar and Alcor were photographed in March 1857 at Harvard College Observatory on wet collodion. The 1874 transit of Venus was also widely photographed on collodion plates as well as daguerrotypes. The collodion plate at right was taken in Japan by Jules Janssen (1824-1907), later director of the Meudon Observatory.
Silver Bromide Dry Emulsions
In 1871 Richard Leach Maddox (at left; 1816-1902), a physician and photographer, produced the first positive dry emulsion for physical development, using gelatin (a transparent animal protein), and then in 1874, J. Johnston and W. B. Bolton made the first negative emulsion for chemical development. By 1878, Charles Bennett had discovered a method by which he could increase the speed (sensitivity to light) of gelatin-silver bromide emulsions (at right) by aging them at 32°C in a neutral medium. This was a most important development for the field of astronomy, since the universe is filled with very faint objects, and astronomers wanted to be able to photograph them without waiting for days and days to get an image on a photographic plate. In 1879, George Eastman (1854-1932) invented a machine to coat plates with emulsion, so that the plates (at left) could be produced in mass numbers, relatively quickly and cheaply. Utilizing the new silver bromide dry emulsion plates, the first good photographs of Jupiter and Saturn were made in 1879-1886, and of comets in 1881 (Tebbutt's comet). A 51 minute exposure of the Orion Nebula was taken in September 1880 by Henry Draper (at right; 1837-1882), a doctor and prominent amateur scientist (and the son of John William Draper), and two years later he took another lasting 137 minutes which revealed the entire nebula and the faintest stars in it. The study of spectra could also be undertaken with the new plates, since they were so much more sensitive to light than those previously. In 1872, the first spectrum of a star-Vega-was taken by Henry Draper. In 1882 Sir William Huggins (who was the first to show that stellar spectral lines could be identified with terrestrial elements, in 1864) took the first spectrum of a nebula (the Orion Nebula), and in 1899 the first spectrum of a "spiral nebula" (now known as a spiral galaxy and much more distant than anything else photographed before) was taken, a 7½ hour exposure taken by Julius Scheiner (at left; 1858-1913) with the Große Refractor of the Astrophysical Institute of Potsdam Observatory. The new kind of plates also brought along with it the era of sky surveying, systematically photographing large expanses of sky. The first sky surveys were done at Harvard during the period 1882-1886, each photograph covering a 15°x15° area of the sky and reaching stars as faint 8th magnitude.
Emulsion Grain Size and Color Sensitivity
Hermann Wilhelm Vogel (at right; 1834-1898), working in Berlin in 1873, accidentally discovered a way to make photographic emulsions sensitive to colors of light other than blue. At the time, green dye was used to soak up reflections off the back side of the glass in a photographic plate. Sometimes this green dye got into the emulsion along the plate edges, and Vogel noticed that the plate in this area was more sensitive to light of a longer wavelength or redder color. This observation was quickly exploited in making new kinds of emulsions which were sensitive at all of the visible colors of light, and by just a year later, Sir William de Wiveleslie Abney (1843-1920) was able to put together an entire optical solar spectrum, from violet to infrared. During the first couple of decades of the twentieth century, C. E. Kenneth Mees (1882-1960) at Eastman-Kodak made outstanding improvements in emulsions and spectral sensitivity. Mees grew particularly interested in the astronomical applications of these new emulsions and so he formed a partnership with several observatories in developing new ways to satisfy their needs, and insisted that Eastman-Kodak provide these plates to astronomers at cost.
Eastman-Kodak and Hypersensitization
After years of research, it has been concluded that different methods of hypering plates yield different results. For instance, the method of pre-exposure involves flashing a light on a plate before the actual exposure is taken for the purpose of reducing the total exposure time of the plate. Thus, image specks will form more quickly and be more stable against decay, so subsequent light is absorbed efficiently. Cooling a plate during exposure, as discovered by Talbot, works by increasing the lifetime of the silver atoms liberated (by the incoming photons) from the silver halide crystals. These silver atoms then survive for long enough to aggregate to make developable latent-image specks. Plates also are baked in nitrogen, oxygen, or just air before exposure. The result is a reduction of the reciprocity inefficiencies (the best improvements are seen for the nitrogen bake, with the least seen from baking in air). Another technique involves soaking a plate in nitrogen or hydrogen gas at room temperature. This helps to drive out the oxygen and water present in the gelatin. Emulsions to absorb infrared light have also been developed, but they are much more sensitive to heat and so much more delicate. However, they can also be hypersensitized, in this case by placing them in a high humidity, oxygen-free environment. For example, they are usually hypered in a bath of distilled water, which results in a gain in speed, or else a bath of ammonia or silver nitrate solution, which helps to remove bromide or iodine ions in solution in the gelatin. The removal with a silver niteate solution will dramatically increase the sensitivity of the IR emulsion.
Newer Photographic Techniques
All photographs suffer from some degree of granulation due to effects in our own atmosphere and also from irregularities in the emulsion itself. A technique for removing these imperfections was invented in the middle part of the twentieth century. If an astronomer can take several images almost simultaneously, each of which presumably would have slightly different granulations, they could then superimpose or "stack" the images and thus remove any irregularities which are not seen in all of the images. This technique is displayed in the series of images of NGC 4672 by David Malin, at right. A method was also developed for detecting very faint and extended objects such as nebulae, which are often not noticed in traditional photographs because they blend into the background light. However, by superimposing the glass photographic negative onto a positive print which was made from light of a different color, astronomers can easily see, for example, blue stars as black spots with white halos around them and red stars as white stars with black halos around them. This contrast more easily allows astronomers to detect nebulae and other faint objects. Additional techniques, such as Photographic Amplification and Unsharp Masking, have allowed some of the lowest surface brightness objects to be discovered, including the giant low surface brightness spiral galaxy Malin.